It has already installed too far. Have you rebooted the machine since the infection started? If yes download superantispyware from here. http://www.superantispyware.com/ install and run a scan. remove everything it finds, restart and then install malwarebytes from http://www.malwarebytes.org and do the same.

If you have not restarted it yet. Hold your power button in for 5-6 seconds to force a cold shutdown. turn it back on and immediately start tapping the F8 key until you get to the startup options screen. There select "last known good configuration" using your arrow keys and press enter twice.

After your boot, you will have a small window of time to get Malwarebytes installed and running. Your Machine is still infected, but the infection should not be in memory.